Add keymanager builder for ePBS

[yorickdowne]

What I did

Support the keymanager builder API. https://ethereum.github.io/keymanager-APIs/?urls.primaryName=dev#/Builder%20Config/setBuilderConfig

It has a lot more options than graffiti/gas, which are just a value each. Yet most users will at most want to set some urls and a min-bid, and maybe boost factor.

To handle that, I expose the simple path, then have a way to print it all to --json and get it --from-json, where a user could have a fully formed builder config.

Added 0xpubkey1,0xpubkey2 form for validator pubkeys

builder URLs right now have no pubkeys in the simple form. I need to see how pubkeys will be used with Glamsterdam before making a decision, that would be a follow-up PR. I expect we won't see what builders do until Hoodi.

Follow-up PRs are:

• Possibly [pubkey@]url form for builders, if pubkeys become common
• Support 0xpubkey1,0xpubkey2 for all other ethd keys commands where it makes sense, such as gas and graffiti
• Have .env support local, maxprofit and always for EPBS_BUILD_FACTOR, and migrate 100 to always during ethd update, once during transition

PR description human
The code was written by Claude Opus 5